One technique for transmitting a stream of information at a high bit rate consists in using ultra wide band (UWB) pulse modulation whereby a sequence of pulses is sent one characteristic whereof (for example the amplitude, the time position or the phase of the pulses) forms a coded representation of the information to be transmitted. These pulses are transmitted with a predetermined average pulse repetition period (PRP).
A transmission system using these principles is described in the patent application EP 1 482 648, for example.
That document proposes, when electromagnetic signals are received, to transpose the analog signals received into the baseband by means of mixers before converting them into digital signals to be processed by means of samplers.
On this point, and in contrast, the patent application EP 1 298 811 proposes to sample the electromagnetic signal as soon as it is received, following simple processing by means of an input stage that is linear in frequency and essentially comprises a low-noise amplifier, with no demodulation processing of the signal in the frequency domain to transpose it into the baseband.
Although it avoids the presence of mixers, this solution involves sampling at a very high frequency in order to comply with the well known Nyquist criterion in order always to recover the whole of the signal in digital form for processing.
Because of this constraint, the use of this solution involving direct sampling of the signals has appeared inappropriate in the case of transmission of streams of information at a lower bit rate, for example from 1 kbps to 10 Mbps. The solution using mixers, as described in the patent application EP 1 482 648, very greatly reduces the sampling frequency used in this case.
Clock drift between two pulses is also a problem in the case of relatively low bit rates: with a standard clock, the shift between two pulses may lead to a loss of synchronization that prevents correct correlation of the signal. (For example, for a typical bit rate of 250 kbps, the duration separating two pulses is typically of the order of 4 μs, during which a standard clock with a slippage of 20 ppm drifts by 80 ps, whereas the correlation of a pulse with a center frequency fC equal to 4 GHz necessitates for synchronization an accuracy of the order of 1/(4fC), i.e. 60 ps).